Gate drivers for power transistors of a power or output stage of Class D audio amplifiers are well-known in the art. The power transistors often comprise N-channel field effect transistors such as NMOSs or IGBTs which are popular semiconductor components for the purpose due to their small ON resistance for given footprint or area consumption on a semiconductor substrate. Since the manufacturing costs of a semiconductor substrate are closely linked to its area, area reduction is an effective manner to reduce costs.
However, the design of suitable gate drivers for such N-channel field effect transistors is challenging for various reasons such as a need to raise an instantaneous gate voltage considerably above a drain voltage of the N-channel field effect transistor during operation of the power stage. The high instantaneous gate voltage is needed to turn the N-channel field effect transistor fully on. Placing the N-channel field effect transistor in its fully on-state or conducting state allows it to exhibit low on-resistance and minimize conductive power losses. Since the drain terminal of an outmost power transistor of a power stage in a Class D audio amplifier is connected to the highest DC supply voltage immediately available, often in form of a positive DC power supply voltage or rail, the instantaneous gate voltage must be raised considerably above this highest DC supply voltage for a duration of the on-state or conducting state of the power transistor in question. Bootstrap techniques and circuitry are known in the art for generating such high gate voltages in each gate driver. However, these rely on a pre-charged capacitor for supplying the gate drive voltage to the N-channel field effect transistor of the power stage. When the pre-charged bootstrap capacitor is connected to a gate terminal of an N-channel field effect transistor, its voltage is significantly reduced by the intrinsic gate capacitance of the N-channel field effect transistor due to charge sharing unless the bootstrap capacitance of the bootstrap capacitor is much larger such as 10 or 20 times larger than the intrinsic gate capacitance. However, the intrinsic gate capacitance of suitable N-channel field effect transistors for many types of power stages may be quite large such as several hundred pF, which leads to impractically large capacitance values for integrated bootstrap capacitors of acceptable dimensions, i.e. capacitance values ranging from several nF to more than 20 nF following the above-mentioned rule of thumb. As an alternative, the bootstrap capacitor can be provided externally to a semiconductor die holding the gate driver or drivers. However, this solution is undesirable since power stage topologies such as multi-level H-bridge power stages typically comprises a plurality of cascade or stacked power transistors with associated gate drivers that each need an external bootstrap capacitor. Such a plurality of external bootstrap capacitors adds to the costs of a complete Class D amplifier solution, requires allocation of valuable printed circuit board space and presents a potential reliability hazard.
Accordingly, gate drivers for power transistors, in particular N-channel field effect transistors, capable of raising the gate voltage above the positive DC power supply voltage or rail with a minimal need for external capacitors are highly desirable. In addition, a high power efficiency of the gate driver would be a significant advantage in numerous applications such as class D audio amplifiers for portable and/or battery operated communication and entertainment equipment such as mobile phones, MP3 players etc.